New numerical modeling suggests that a hypothetical moon around Venus could be lost through tidal evolution, the slow exchange of spin and orbital motion caused by gravitational tides. The outcome changes sharply with the planet's starting spin and with the way tidal behavior is represented in the model. The modeled conditions that both remove the moon and substantially slow Venus's spin are narrow.
The paper asks whether a moon formed around Venus through a giant impact could survive to the present day. Its result is a conditional model outcome, not a reconstruction of Venus's actual history.
A sharp boundary between survival and destruction
In a fiducial constant-Q run, a one-lunar-mass satellite started five Venus radii from the planet. With QV = 50, it survived the full 4.5-billion-year calculation when Venus's initial spin period was up to 12 hours. At 15 hours, the modeled moon was destroyed after about 0.97 billion years. At 24 hours, destruction came within about 1 million years.
That boundary is not universal. It shifts with the initial orbit, the moon's mass and the model treatment. In the same comparison, raising QV from 50 to 100 extended lifetimes by about a factor of two, while QV = 10 shortened them.
The sweep covered initial spin periods from 5 to 100 hours, moon masses from 0.01 to 10 lunar masses, QV values from 10 to 100, initial eccentricities from 0 to 0.5, and starting distances of five Venus radii, with additional runs from 3.5 to 25 Venus radii. Eccentricity here is the measure of how far an orbit departs from a circle.
The researchers integrated coupled equations with fourth-order Runge-Kutta and adaptive timesteps set to resolve 1% fractional changes in Venus's spin and the moon's orbital distance. Runs ended when the moon crossed the Roche limit used as a loss condition, exceeded the critical stability radius, or reached 4.5 billion years.
At the fiducial setup, the moon's tidal torque was about 3 million times stronger than the Sun's. In that modeled case, the moon, rather than the Sun, dominated the spin evolution.
The assumptions can change the answer
The two tidal models gave different results for larger satellites. For a moon twice the lunar mass, constant-Q predicted Roche destruction after 1.7 billion years from an 8-hour initial spin and after 33 million years from a 12-hour spin. The constant-time-lag model, known as CTL, predicted survival in both cases. For a five-lunar-mass moon, constant-Q predicted destruction within about 100 million years, while CTL produced a quasi-synchronous equilibrium for initial spins up to 15 hours.
The CTL calculations also showed a loss route linked to eccentricity. At small eccentricity, the modeled eccentricity behavior changed sign when Venus's rotation was about 1.636 times the moon's orbital rate, corresponding to an initial spin period of about 10 hours for a five-Venus-radius orbit. With an 8-hour initial spin, low-mass moons could be lost even when their starting eccentricity was only 0.01.
A possible history, not a proven one
Together, these results create a narrow overlap between two goals: losing a moon and substantially despinning Venus. In the model, cases that lose the moon leave Venus rapidly rotating, while surviving cases occupy the more despun part of the results.
The authors point to a narrow post-impact spin window near 12 to 15 hours as potentially relevant to Venus's history. In that window, a lunar-mass moon is marginally destroyed on billion-year timescales. Faster and slower post-impact regimes fall into different survival zones.
As a methodological check, the same spin-orbit calculations were applied to the Earth-Moon system. They produced a lunar recession rate of 3.69 centimeters per year, compared with an observed 3.82, an agreement within 3%. They also produced a length-of-day increase of 2.05 milliseconds per century, versus about 2.3 observed, within 11%.
That check tests the spin-orbit coupling, not Venus's detailed past. The study uses two simplified descriptions of tidal response and says neither is fully realistic for rocky bodies. It also warns that constant-Q lifetimes near synchronous reversal are sensitive to numerical treatment, while CTL is more robust near the survival boundary.
The calculations include gravitational body tides from the moon and Sun, but omit atmospheric thermal tides. The authors call that omission conservative and say including the full effect would mean shorter satellite lifetimes.
Any extension to exoplanets is therefore qualitative. For a Venus analog at 0.1 AU around a 0.3-solar-mass M dwarf, the model's critical spin period is about ten times smaller than Venus's, and moon survival is described as effectively impossible for plausible post-impact spins. The paper cautions that these quantitative results are Venus-specific.
Paper data and sources
Original title: Tidal Demise: The Evolution and Fate of a Hypothetical Venus Moon
Authors: Stephen R. Kane, Franck Selsis, Jeremy Leconte, Sean N. Raymond
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
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